The present invention relates generally to an oxygen sensor and, more particularly, to an oxygen sensor of the type designed to detect the concentration of oxygen in exhaust gases discharged from internal combustion engines.
One conventional oxygen sensor known to be incorporated in an exhaust pipe of an internal combustion engine includes a protective cover placed around a sensor element to make an exhaust gas flow uniform, as set forth in Japanese Utility Model Laid.Open Publication No. Hei. 1-169350. Another oxygen sensor assembly having a double-structure type protective cover is known from U.S. Pat. Nos. 4597850 and 4683049.
However, problems with such conventional oxygen sensors are that in the case of the single protective cover, an amount of water, which is generated when the internal combustion engine is started up, is likely to reach the sensor element location through a gas inlet hole provided in the protective cover, and in the case of the double protective cover, an amount of water is deposited through a gas inlet hole in the outer protective cover onto the surface of the inner protective cover due to surface tension and then remains stagnant between the outer and inner cylinders. As the internal combustion engine is accelerated, the water stagnating between the inner and outer cylinders due to surface tension is so likely to reach the sensor element location through the gas inlet hole in the inner cylinder.protecting cover that the sensor element may crack or break, although not often. Especially where a heating type sensor with a built-in heater is used with an internal combustion engine, the sensor element is considerably heated within 20 to 80 seconds after it has been started up. When water strikes upon the sensor element in this state, it may often crack and, in the worst case, break down.
An object of this invention is to solve such problems by the provision of an oxygen sensor assembly designed to keep the sensor element in good order even when it is exposed to water scattered in an exhaust pipe at the time of engine start-up.